Process for the purification of a gas by scrubbing -- Venturi column for carrying out said process

ABSTRACT

Gas which is contaminated with liquid and/or solid is purified using a vertically oriented Venturi column in conjunction with a sound field. At the throat of the Venturi column, a scrubbing liquid is atomized and injected in a direction substantially perpendicular to the flow of the contaminated gas. A sound field is generated at the throat in order to cause the contaminate to shift relative to the vesicles of the scrubbing liquid.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the purification of a gasby scrubbing and to a device, namely a vertical-axis Venturi column, forcarrying out said process.

The present invention in fact constitutes an improvement to thescrubbing processes for the purification of gases contaminated by liquidand/or solid particles (aerosols), wherein a scrubbing liquid isatomized in co-current with the contaminated gas. The vesicles of saidscrubbing liquid intercept the contaminating particles carried by saidgas. Such processes have been carried out particularly in devices suchas Venturi scrubbers or vertical-axis Venturi columns of circularcross-section. Said scrubbing liquid is injected into the gas to bepurified, upstream of or at the entrance of the throat of theVenturi(s).

2. Description of Related Art

According to the prior art, processes for the purification of gasescontaminated by liquid and/or solid particles, in a vertical-axisVenturi column, have been particularly described and carried out, saidprocesses comprising:

at the throat of a Venturi of circular cross- section, the pneumaticatomization of a scrubbing liquid in a rising stream of the gas to bepurified, said scrubbing liquid being injected in a directionsubstantially perpendicular to that of said stream; and

downstream of the throat of the Venturi, the elimination of the vesiclesof scrubbing liquid entrained by said gas stream and laden withcontaminants.

Elimination of the vesicles is understood as meaning any of the chemicalengineering operations whose purpose is to eliminate from a gas at leastsome of the particles, in this case vesicles, suspended in said gas.

Such scrubbing processes for the purification of gases in avertical-axis Venturi column equipped with a Venturi of circularcross-section are described in greater detail later on in the presenttext with reference to FIG. 1. As indicated above, the present inventionconstitutes an improvement to such processes.

According to the prior art, acoustic processes have also been proposedfor the purification of gases contaminated by solid and/or liquidparticles. The shifting of the contaminating particles which is due tothe acoustic field causes two phenomena contributing to the purificationof the gas:

the disappearance of said particles by precipitation on the walls whichlimit the agglomeration volume; and

the disappearance of the finest particles by collision with coarserparticles in the case of a polydisperse aerosol. The resulting change inthe size distribution of the aerosol affords a significant reduction inthe component for which the conventional separation processes are theleast effective.

There are very few industrial apparatuses which utilize such acousticagglomeration. It is possible to mention a few applications, mainly onthe pilot scale, in the world, and a few laboratory-scale embodiments inFrance, especially the one described by C. A. Stokes, Sonicagglomeration of carbon black aerosol, Chem. Eng. Progr. 46(8): pages423-432(1950).

Acoustic agglomerators principally consist of a large coagulationchamber (6 to 9 m in height) in which the treated gases, circulating atlow velocity (2 to 5 m/s), are subjected to the action of an acousticwave emitted by a sonic generator, which is generally a siren inindustrial applications and an electric loudspeaker in pilot plants.(The residence time of the gases inside said chamber must besufficient.)

At the exit of the agglomeration chamber, where the acousticconditioning takes place which increases the median diameter of thecontaminating particles so as to facilitate their subsequent capture,the gases pass through a dust-eliminating or vesicle-eliminating devicesuch as a cyclone, multicyclones, packed column or filter medium.

The phenomena of acoustic precipitation on the walls by means of a fieldwith a high intensity of the order of W/cm² make a major contribution tothe purification of the gases in this type of apparatus, mainly in thecase of vesicle elimination by means of a packed bed, sieve or filtermedium.

Inside an acoustic agglomeration chamber, it is possible to spray incountercurrent with the gas to be treated. In such a case, the drops ofsprayed liquid entrain the contaminating particles which have previouslybeen enlarged by acoustic agglomeration.

Such processes for the purification of gases by acoustic agglomerationof the contaminating particles they contain are described in greaterdetail later on in the present text with reference to FIG. 2.

Such acoustic processes have the following disadvantages:

they involve a greater energy consumption than an electrofilter;

their efficacy is negligible in the case of contamination by amonodisperse aerosol or an aerosol with a small standard deviation ofthe particle size distribution;

the required acoustic intensity is high;

the bulk of the device is considerable in view of the low velocity ofcirculation of the gases; and

acoustic insulation is sometimes essential to prevent noise pollution.

SUMMARY OF THE INVENTION

According to the invention, the phenomena of acoustic agglomeration areutilized to advantage in a process of purification by scrubbing in avertical-axis Venturi column.

Acoustic agglomeration is used synergistically with the phenomena ofturbulent or brownian coagulation which take place in Venturi-typescrubbers, where the scrubbing liquid is atomized in the contaminatedgas, advantageously in the form of a monodisperse aerosol of highnumerical concentration (containing a large number of vesicles per unitvolume).

The process of the invention therefore combines acoustic agglomerationwith the scrubbing of a gas in a Venturi column by the pneumaticatomization of vesicles of the scrubbing liquid at the throat of theVenturi. Such a combination was not at all obvious insofar as it wasestablished that the phenomena of sonic coagulation were found to be ofinterest only with long residence times (≧1 s) and also a relativelyhigh applied acoustic intensity (about 1 W/cm²).

Now, the acoustic agglomeration process of the invention improves aconventional process of purification by scrubbing in a Venturi columnhaving residence times of the order of a few tenths of a second and byusing a relatively low acoustic intensity (from 0.1 to 0.5 W/cm²).

Said process according to the invention for the purification of gasescontaminated by liquid and/or solid particles is therefore carried outin a vertical-axis Venturi column and comprises:

at the throat of a Venturi, the pneumatic atomization of a scrubbingliquid in the rising stream of gas to be purified, said liquid beinginjected in a direction substantially perpendicular to that of saidstream; and

downstream of said throat of the Venturi, the elimination of thescrubbing vesicles entrained by said gas stream and laden withcontaminants.

Said process is characterized in that a sound (or acoustic) field isgenerated at said throat of the Venturi in order to cause the liquidand/or solid contaminating particles to shift relative to the vesiclesof scrubbing liquid.

The process of the invention therefore utilizes the conventionaltechnique for the generation, advantageously at high concentration, ofvesicles of scrubbing liquid by pneumatic atomization in the throat of aVenturi. This technique is combined with that of acoustic agglomeration:the vesicles of scrubbing liquid of relatively large diameter are notshifted by the acoustic field and serve as centers of agglomeration forthe contaminating particles of very much smaller diameter.

The process of the invention is particularly suitable for thepurification of gases contaminated by particles with a diameter ofbetween 0.2 and 20 μm (monodisperse or polydisperse aerosols), theacoustic agglomeration improving the capture of the particles of smalldiameter (diameter of less than 5 μm).

In said process, the scrubbing liquid is advantageously atomized in theform of a monodisperse aerosol whose vesicles have a diameter of between10 and 1000 μm, advantageously of the order of 100 μm. Said diameter isoptimized as a function of the characteristics of the treated aerosol.As far as the concentration of said vesicles of scrubbing liquid isconcerned, it can be specified here by way of indication that,advantageously, according to the invention, vesicles with a diameter ofbetween 40 and 80 μm. are generated at a rate of about 5000 per cm³.

Those skilled in the art are capable of adjusting the atomizationcharacteristics so that the mean distance between the generated vesiclesis of the order of magnitude of the amplitude of the vibrationalmovements of the contaminating particles which are due to the acousticfield, said adjustment being necessary to optimize the process of theinvention.

The acoustic or sound field generated, according to the invention, atthe throat of the Venturi can be generated upstream or downstream of thepneumatic atomization of the scrubbing liquid.

As specified above, the scrubbing vesicles laden with contaminants areentrained by the rising gas stream and are eliminated downstream of thethroat of the Venturi, generally downstream of the Venturi itself, whichis advantageously extended by an agglomeration chamber, making itpossible to increase the residence time of the aerosol (gas+scrubbingvesicles+contaminating particle) in the sound field.

The elimination of the vesicles must not cause resuspension, in the gasstream, of the contaminating particles trapped in the vesicles ofscrubbing liquid. The velocity of the gas stream and/or the shapes ofthe impingers are optimized for this purpose.

It is advantageous to ensure that, at said impingers, the liquidresulting from the elimination of the vesicles trickles rather thandrips.

The technology of the invention-atomization of a scrubbing liquid at thethroat of a Venturi+generation of an acoustic field at said throat--hasthe following advantages:

the contribution of acoustic agglomeration to the stopping power (orefficiency) in terms of the overall incident contamination is small, butit acts essentially on the particle fraction of smallest diameter, forwhich the scrubber would be ineffective in the absence of the acousticfield. This specific action thus increases the overall efficacy of theapparatus considerably;

it permits the agglomeration of monodisperse aerosols or aerosols with anarrow dispersion, which was impossible with the acoustic agglomeratorsof the prior art;

during their relaxation time, which is generally of the order of 0.01 s,the centers of agglomeration formed by the vesicles generated byinjection at the throat of the Venturi--in a direction more or lessperpendicular to that of the gas stream to be purified--have a velocityrelative to the gas to be purified. This results in a substantial flowof particles through their agglomeration volume, ensuring a highcollision efficacy for a fraction of the residence time in the apparatus(start of said residence time);

control of the flow rate of liquid injected at the throat results incontrol of the numerical concentration of the coarse particles acting ascenters of coagulation, of which the probability of collision, i.e. thevelocity of acoustic agglomeration, is a direct function. Thepossibility of generating a high numerical concentration of centers ofagglomeration makes it possible on the one hand to shorten the residencetime and on the other to reduce the acoustic intensity applied;

the compactness of the apparatuses and their reduced dimensions comparedwith the acoustic agglomerators of the prior art enable high frequenciesto be used without a notable drop in intensity as a function of thedistance from the origin.

The process of the invention, the value of which will already havebecome apparent to those skilled in the art, is suitable for thepurification of gases contaminated by liquid and/or solid particles, itbeing possible for said gases to be generated in the nuclear industry,chemical industry, car industry, iron and steel industry, etc.

The value of the process of the invention in the nuclear industry willbe given very particular emphasis here. In this industry, it is in factcommon to be faced with the problem of the separation of liquidaerosols, which constitute the main vehicle of the active contaminationof gases to be purified. The efficacy of this separation must be such asto attain the highest possible decontamination factors while at the sametime respecting the particular constraints peculiar to equipment workingin an active medium. The process of the invention is particularlysuitable for the treatment of radioactively contaminated gases such as:

the process gases,

the vapors resulting from concentration of the liquid effluents, and

the fumes from incineration of active waste (graphite).

As indicated above, it can also be carried out to advantage in thechemical industry (especially for the purification of acid mistscontaminated by H₂ SO₄ or H₃ PO₄, or gaseous effluents from thedyestuffs industry contaminated by TiO₂, TiSO₄, TiCl₄, etc.), in theiron and steel industry (especially for the purification of fumes fromthe electric furnaces or brown smoke from the converters), in the carindustry (especially for the purification of oil mists or aircontaminated by paint aerosols), etc.

The value of the process of the invention in protecting the environmentfrom urban pollution may be mentioned as a further illustration. Theprocess is advantageously used to treat the fumes resulting from theincineration of household refuse or to purify the air for ventilatingurban tunnels.

The present invention further relates to a device specially designed forcarrying out the process described above.

Said device is a vertical-axis Venturi column of the same type as thoseof the prior art. It has in particular, in its bottom part, at thethroat of a Venturi, means for pneumatic atomization of the scrubbingliquid in the rising stream of the gas to be purified, said meansinjecting said scrubbing liquid in a direction substantiallyperpendicular to that of said gas stream, and in its top part,downstream of said throat and generally downstream of the Venturiitself, means for elimination of the vesicles of scrubbing liquidentrained by said gas stream and laden with contaminants.

Characteristically, it also has means for generating a sound or acousticfield at said throat of the Venturi. As indicated above, said sound oracoustic field is intended to cause the contaminating particles--liquidor solid particles of small diameter--to shift relative to the vesiclesof scrubbing liquid--of larger diameter--and thus to enable saidcontaminating particles to be captured by said vesicles.

The means for generating the sound field are located upstream ordownstream of the means for atomizing the scrubbing liquid.

The means for eliminating the vesicles of scrubbing liquid laden withcontaminants advantageously consist of impingers, the geometry of whichcauses the impacted vesicles to trickle along walls. Thus an attempt ismade to prevent the contaminating particles "trapped" in the vesicles ofscrubbing liquid from being freed at the vesicle elimination stage.

In a first variant, the device of the invention has a cylindricalVenturi. More precisely, the plane cross-section of said Venturi,perpendicular to the flow of the gas stream, is circular.

According to the invention, means for generating a sound field aretherefore combined with such a Venturi. Said means consist of anacoustic generator, advantageously of the Galton whistle (or "Kurkingschok jet") type, mounted in a resonance tube and fed with compressedair. Said resonance tube acts as a nozzle which uses said compressedair, after actuation of the whistle, as a driving gas injected into thethroat of the Venturi. Such an arrangement makes it possible to reducethe pressure losses of the Venturi column according to the invention byusing the Venturi as an injector. Furthermore, the expansion of thecompressed air causes the contaminated gases to cool by mixing withthem, which favors enlargement of the contaminating particles bycondensation.

In a second, preferred variant of the invention, the device has anon-cylindrical Venturi. Said device is a plane Venturi column withacoustic agglomeration. More precisely, the plane cross-section of theVenturi, perpendicular to the flow of the gas stream, is rectangular.Its length or width is limited by acoustic constraints pertaining to thewavelength used (or to the shape of the sonic generator used). It is forthis reason that, according to the flow rate of gas to be treated, theVenturi column of the invention can have one or more identical, planeVenturis mounted in parallel.

Such a geometry gives the column of the invention superiorcharacteristics to those of the Venturi column with a Venturi ofcircular cross-section, as regards the bulk of the apparatus, thequality of the atomization and the power of the sonic generator. Theadvantages gained as far as the bulk and atomization quality areconcerned are due to the ratio of the area of the cross-section to thediameter (or equivalent diameter), which, while constant in the case ofa circle, is an increasing function of the length/width ratio in thecase of a rectangle.

According to the invention, means for generating a sound field aretherefore combined with such a plane Venturi or such plane Venturis.Said means can consist of different types of acoustic generators.

A series of acoustic generators of the type specified above, namely aGalton whistle or Kurking schok jet, can be combined with each of saidVenturis.

In one variant of the invention, it is also possible to equip eachVenturi with two vibrating-jet whistles of which the orifices of theresonance volumes may or may not be opposite one another.

In this variant, the throat of the plane Venturi can be provided,upstream or downstream of the scrubbing liquid injection nozzles, withcavities of equal volume which are arranged symmetrically relative tothe axial plane of the jet of gas to be purified (symmetrical cavitiesfacing each other) and function like HELMHOLTZ resonators. Said cavitiesare provided with a knife, enabling them to function like the cavity ofa vibrating-jet whistle.

The symmetrical cavities and the throat of the Venturi in the region ofthe sonic generator have to be separated by partitions which limit thelength of the knives to a size which is no greater than the wavelengthof the sound emitted by the cavities. In fact, beyond this length, thereis a drop in efficiency due to a lack of simultaneity in the breaking ofthe gas stream on the edge of the knife.

In the case of a conventional whistle, whose plane jet of gas is verythin, a small angular deviation causes substantial pressure variationsin the resonance cavity. By contrast, in the case of the sonic generatorin question, where the jet of air is relatively thick, the "sensitive"region of the jet, near the knife, is situated in the laminar zone ofthe flow, in which the velocity is less than the mean delivery velocity.The velocity of the gas circulating in front of the cavity orifice maytherefore be insufficient to trigger the resonance.

This disadvantage is overcome by advantageously adding, upstream of saidcavities, an exciting device consisting of two parallel rods of circularcross- section, the diameter of which (about 4 mm) is calculated so asto generate Karman vortices with a frequency equal to that of the cavityof the resonators when the velocity at the throat is slightly less thanthe nominal operating velocity. This exciting device increases theacoustic intensity which would be due to the resonance cavities if theywere to be used on their own.

The fact that the orifices of the resonance cavities are opposite oneother and are a short distance apart in the gas stream causessynchronization of their frequency by mutual influence. Under theseconditions, the variations in pressure loss in the throat of the Venturiin line with these orifices, due to the turbulences caused by thesynchronized entry or exit of air at the two orifices, generate,downstream of the generator, a pulsed flow comparable to that of asiren. This pulsed flow enables the nozzles to function in accordancewith the same principle as those of vibrating-nozzle atomizers and togenerate vesicles of scrubbing liquid whose diameter is controlledbetter than that of the vesicles obtained by simple pneumaticatomization.

In this same variant of the invention, the acoustic generator canconsist of two vibrating-jet whistles of different natural frequency, ofwhich the orifices of the resonance volumes are not opposite one another(cavities of different volume). The consequent increase in the number offrequencies emitted, on account of the harmonics, broadens the activecapacity of the process.

The propagation of the sound waves upstream of the generator makes itpossible to precondition the contaminating particles. Said sound fieldcauses coagulation of said particles, i.e. an increase in their meandiameter, before they pass through the "filter medium" which is formed,downstream, by the array of scrubbing vesicles suspended in the gas.

Such sonic generators absorb their energy from the gas stream to bepurified. Their operation generates pressure losses. These pressurelosses are additional to those due to atomizing the scrubbing liquid,bringing it to the required velocity and raising it from the atomizationlevel to the level of the impingers.

Such pressure losses can be at least partially compensated by equippingthe Venturi column according to the invention, upstream of the twofacing or non- facing whistles, with means for injecting a driving gas,which is advantageously a pressurized gas (pressurized air or vapor)ejected at supersonic velocity. This introduces a small flow of gas at ahigh velocity.

The necessary flow of driving gas is advantageously controlled by thepressure loss in the apparatus.

Said driving gas also serves to maintain a velocity at the throat whichis compatible with the operation of the sonic generator.

There is a further advantage in using a Venturi or Venturis ofrectangular cross-section in the Venturi columns of the invention. Thegeometry of the divergent portion of such Venturis can be optimized sothat they function as acoustic flares (thereby optimizing thedistribution of the sound field in the coagulation chamber). For thispurpose, the cross-section of said divergent portion variesexponentially with the distance from its origin.

The invention will now be described with reference to the attachedFigures.

BRIEF DESCRIPTION OF THE DRAWINGS

The characteristics of the process and device of the invention arespecified with reference to said Figures without in any way implying alimitation.

FIG. 1 shows a Venturi column according to the prior art.

FIG. 2 diagrammatically represents an acoustic agglomerator according tothe

FIG. 3 shows a Venturi column equipped, according to the invention, withan acoustic agglomerator or sonic generator (1st variant).

FIG. 4 is a cutaway view of the acoustic agglomerator with which theVenturi column of FIG. 3 is equipped.

FIG. 5 shows a Venturi column equipped, according to the invention, withan acoustic agglomerator or sonic generator (2nd variant).

FIG. 6 is a cutaway view along 6--6 of FIG. 5.

FIG. 7 is an enlarged view of a detail of FIG. 6, namely the sonicgenerator.

FIG. 8 is a cutaway view of said generator along 8--8 of FIG. 7.

The operation of a Venturi column according to the prior art, useful forscrubbing contaminated gases G, is described below with reference toFIG. 1.

Said contaminated gases G enter the reservoir (2) through the tube (1);they then enter the throat (3) of the Venturi, where they generallycirculate at a delivery velocity of the order of 80 m/s. This velocityenables them to atomize the scrubbing liquid injected at the entrance ofthe throat through the injector nozzles (19).

These nozzles are fed with "charge" through the tube (13) from theliquid contained in the lower part of the reservoir (8), the level ofwhich is kept constant by the overflow (10). This liquid, which isrecycled to the injector nozzles (19), originates from elimination ofthe vesicles from the gases by the impinger of annular cross-section(7).

To limit the concentration of contaminant in the recycled liquid, amake-up flow of scrubbing liquid, the value of which depends on theconcentration of contaminant in the treated gases, is mixed with therecycled flow upstream of the nozzles (19); this flow is introduced intothe tube (13) via (15). The excess contaminated liquid originating fromthe overflow (10) flows into the reservoir (2) through the dip tube(14).

The level of the liquid in the lower reservoir (2) is kept constant bythe overflow chamber (16), in which the pressure is equilibrated withthat in the "roof" of the upper reservoir (8) by the tube (17); thecontaminated liquid originating from the overflow chamber (16) isdischarged through the tube (18).

In the divergent portion (4), the decrease in the velocity of the gasesconverts a fraction of their dynamic pressure to static pressure. Thisdivergent portion (4) is extended by the cylindrical volume (5), whichincreases the contact time of the contaminated gas with the vesicles ofscrubbing liquid and favors agglomeration by differential sedimentation.

In the top part of the volume (5), the convergent part of annularcross-section (6) restores the velocity of the gases. Their projectiononto the impinger (7) causes the capture of the vesicles produced by theatomization of the scrubbing liquid, which have agglomerated with mostof the contaminating particles.

The gas purified by elimination of the vesicles in the impingers escapesinto the reservoir (8) and leaves the apparatus through the tube (11).The liquid from vesicle elimination flows into the bottom end of (8),which is connected to the recovery trough (9) by the tube (12).

FIG. 2 diagrammatically represents an acoustic agglomerator according tothe prior art. It functions in the manner specified below. Thecontaminated gases G enter a vertical cylindrical reservoir (102)through the tube (101). Said reservoir constitutes the agglomerationchamber for the contaminating particles. Inside said reservoir (102),the gases are subjected to the action of an acoustic wave emitted by thesiren (103) fed with compressed air through the tube (104). Afteracoustic conditioning in said reservoir (102), the gases pass through adust eliminator, which here consists of a multicyclone comprising nidentical cyclones (105).

The purified gases leaving said cyclones (105) escape through the tube(106), while the dusts (or the liquid from vesicle elimination) aredischarged through the tubes (107).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 3 and 4 illustrate the first variant of the invention, namely aVenturi column with sonic generator, said Venturi having a circular,plane cross-section (perpendicular to the flow of the gas stream).

Said Venturi column is of the same type as those known according to theprior art. The description of the apparatus and fluid circuit isidentical to that given above with reference to FIG. 1.

Said FIGS. 1 and 3 carry the same reference numbers for the samecomponents.

Characteristically, the column of the invention has, upstream of thescrubbing liquid injector nozzles (19), a sonic generator (20) fed withcompressed air through the tube (21).

Said sonic generator (20) (or acoustic generator (20)) is shown indetail in

FIG. 4. It consists of a whistle (210) of the Kurking schok jet type,mounted in a resonance tube (211). Compressed air is injected at (212)in order to operate said whistle (210). It is ejected at (213) in thethroat (3) of the Venturi. It acts as a driving gas.

FIGS. 5 to 8 illustrate the second variant of the invention, namely aVenturi column with sonic generator, said Venturi having a rectangular,plane cross-section (perpendicular to the flow of the gas stream). Thisis referred to as a plane Venturi column with sonic agglomerator.

The apparatus represented in the diagram has two Venturis of width L inparallel.

The contaminated gases G enter the lower, horizontal-axis reservoir (32)through the tube (31), said reservoir distributing the flow of saidgases uniformly in the throat of the plane Venturis, the lower part (35)of which acts as a mixer for the injector function provided by theVenturis.

The driving gas (compressed air) is injected into the mixer through thenozzles (34), which are equal distances apart on a manifold (33). Thekinetic energy produced by the mixing of the driving gas, ejected atsupersonic velocity, with the contaminated gas serves to compensate forthe pressure losses due to the sonic generator, the atomization, thevelocity adjustment and the raising of the scrubbing liquid.

After mixing with the driving gas, the contaminated gases pass throughthat segment of the throat of the Venturi which is occupied by the sonicgenerator (39). This generator is described in greater detail a littlelater on with reference to FIGS. 7 and 8.

The gases leave the segment of the throat occupied by the sonicgenerator (39) at a "delivery" velocity of the order of 80 m/s andatomize the scrubbing liquid injected through the nozzles (40) togenerate vesicles, the diameter of which is advantageously of the orderof 60 μm..

That segment of the throat of the Venturi which is situated downstreamof the nozzles (40) serves to mix the aerosol produced by theatomization of the scrubbing liquid with the contaminated gasessubjected to the sound waves. The nozzles (40) are fed with "charge",via the manifolds (51) and the tubes (50), from the liquid contained inthe lower 30 part of the reservoir (48), whose level is kept constant bythe overflow (47). This recycled liquid originates from the eliminationof vesicles from the gases by the impingers (44).

To limit the rise in the concentration of contaminant in the recycledliquid, a small make-up flow of scrubbing liquid (the value of whichdepends on the concentration of contaminant in the treated gases) ismixed with the recycled liquid upstream of the overflows (47); this flowis introduced into the upper reservoir (48) through the dip tube (45).

The excess contaminated liquid originating from the overflows (47) flowsinto the lower reservoir (32) through the dip tube (52). The level ofthe liquid in the lower reservoir is kept constant by the overflowchamber (54), in which the pressure is equilibrated with that in the"roof" of the upper reservoir (48) by the tube (53); the contaminatedliquid originating from the overflow chamber (54) is discharged throughthe tube (55).

In the divergent portion (41), which may act as an acoustic flare whosecross-section varies exponentially with the distance from the origin ofthe divergent portion, the decrease in velocity converts part of thedynamic pressure of the mixture of contaminated gases and driving air tostatic pressure. The divergent portion is extended by an agglomerationchamber (42) of constant cross-section, which increases the residencetime of the aerosol in the sound field.

At the exit of the agglomeration chamber (42), the gases are brought tothe required velocity by the convergent portions of rectangular, planecross-section (43); the projection, onto the impingers (44), of the jetsof gas leaving said convergent portions causes the capture of thevesicles produced by the atomization of the scrubbing liquid.

The velocity of impact and the dimensions of the impinger are calculatedso as to ensure, with the minimum pressure loss, an efficacy of 100%vis-a-vis the vesicles of scrubbing liquid which have agglomerated withthe contaminating particles.

The gas purified by vesicle elimination escapes into the "roof" of thereservoir (48) through the lateral openings in the sides (46) of theimpinger plates (44) and leaves the apparatus through the tube (49).

By trickling over the inner face of the sides (46) of the impingers, theliquid from vesicle elimination flows into the lower part of thereservoir (48), which is bounded by the four overflows (47).

The propagation of the sound waves in the reservoir (32), upstream ofthe generator, enables the latter to be used as a preconditioning volumefor the contaminating aerosol. In fact, the sound field prevailing inthe reservoir (32) causes coagulation of the contaminating particles,i.e. an increase in their mean diameter, before they pass into thedownstream "filter" medium formed by the array of scrubbing vesicles insuspension.

The energy absorbed by the sonic generator (39) is compensated by theenergy which is supplied by the injectors consisting of Laval nozzles(34), uniformly distributed under the throat of the plane Venturi.

Said sonic generator (39) consists of the two resonance cavities (37) ofequal volume which are arranged symmetrically relative to the axialplane of the jet of gas and function like HELMHOLTZ resonators. Saidcavities (37) are each provided with a knife (38). There arc twoparallel rods (36) of circular cross- section upstream of said cavities(37). These rods (36) generate Karman vortices. Their diameter iscalculated so as to generate such vortices with a frequency equal tothat of the cavity of the resonators when the velocity at the throat isslightly less than the nominal operating velocity. Such a deviceincreases the acoustic intensity which would be due to the resonancecavities if they were to be used on their own.

FIGS. 3-4 and 5-7 show two variants of a Venturi column according to theinvention, in which the means (20 and 39) for generating the sound fieldare located upstream of the means (19 and 40) for atomizing thescrubbing liquid.

As indicated in the present description, the possibility that said meansfor generating the sound field might be located downstream of said meansfor atomizing the scrubbing liquid is in no way excluded. The fineratomization which can thus be obtained proves particularly advantageousfor treating aerosols of low numerical concentration and/or forachieving very high purification levels.

The invention is illustrated by the Example below.

A) We were interested in the purification of a vapor produced by aboiler. Said boiler contains an aqueous solution of a strontium salt.Strontium is used as a tracer. The solution contains 20 g/l thereof.

The boiler, adjusted to a boiling rate of 400 kg/h.m², produces a vaporladen with liquid particles having a diameter of between 0.2 and 20 μm.The mass concentration of the liquid particles suspended in the vapor isabout 2×10⁻³ g/m³.

It is proposed to purify said vapor and quantify the purification bymeasuring the decontamination factor (DF): ##EQU1## mass of contaminantin the incident gas=mass of strontium in the vapor (G) at the entranceof the Venturi column (foot),

mass of contaminant in the emergent gas=mass of strontium in the vaporat the exit of the Venturi column (head).

The maximum decontamination factors (DF) detectable by atomic absorptionspectrophotometry with the measuring means used are 2×10⁵ for strontium.

The vapor to be purified has the following characteristics:

    ______________________________________                                        flow rate (nominal)                                                                         Q       0.173      kg/s                                                                          (#1000 m.sup.3 /h)                           density       ρg  0.598      kg/m.sup.3                                   dynamic viscosity                                                                           η   1.21 × 10.sup.-5                                                                   Pa · s                              temperature   θ 100        °C.                                   pressure      p       10.sup.5   Pa                                           ______________________________________                                    

B) Said vapor is treated in a Venturi column equipped with a cylindricalVenturi, as illustrated in FIG. 1.

The characteristics of said column are as follows:

    ______________________________________                                        Diameter at the throat of the Venturi                                                                 D.sub.C = 0.08 m                                      Diameter of the agglomeration chamber                                                                 D.sub.A = 0.40 m                                      Height of the agglomeration chamber                                                                   H.sub.A = 1 m                                         Overall height of the apparatus                                                                       H.sub.T = 3.6 m                                       ______________________________________                                    

Water (scrubbing liquid) is injected at the throat of the Venturi at arate of 0.5 kg/m³ of treated vapor. The scrubbing vesicles generatedhave a diameter of about 350 μm.

The vapor decontamination factor in a Venturi column according to theprior art is DF=300.

We were able to demonstrate the fact that the purification processcarried out in this way is ineffective with respect to contaminatingparticles with a diameter less than or equal to 1.5 μm.

C) Said vapor is treated under the same conditions (same Venturi column,same parameters pertaining to the scrubbing liquid) but with acousticagglomeration according to the invention (FIG. 3).

The acoustic agglomerator used is a whistle of the Kurking schok jettype with a theoretical acoustic power of 126 W (consumption: 840 W).Said whistle is mounted upstream of the scrubbing liquid atomizationnozzles. It is fed with compressed air (2 bar) at a rate of 0.01 kg/s.

The frequency of the generated wave is 3500 Hz.

The acoustic intensity in the agglomeration chamber is 1000 Wm² (0.1W/cm²).

The vapor decontamination factor in a Venturi column according to theinvention is DF=1000.

I claim:
 1. In a process for the purification of a gas (G) contaminatedby liquid and/or solid particles, in a vertical-axis Venturi column,comprising, at the throat (3) of a Venturi, the pneumatic atomization ofa scrubbing liquid in a rising stream of said gas, said scrubbing liquidbeing injected in a direction substantially perpendicular to that ofsaid stream, and, downstream of said throat, the elimination of thevesicles of scrubbing liquid entrained by said gas stream and laden withcontaminants, the improvement which comprises generating a sound fieldat said throat (3) of the Venturi in order to cause the contaminatingparticles to shift relative to the vesicles of scrubbing liquid.
 2. Aprocess according to claim 1 characterized in that the contaminatingparticles have a diameter of between 0.2 and 20 μm and in that thescrubbing liquid is atomized at the throat (3) of the Venturi in theform of a monodisperse aerosol whose vesicles have a diameter of between10 and 1000 μm.
 3. A process according to characterized in that thesound field is generated upstream of the pneumatic atomization of thescrubbing liquid.
 4. A process according to any one of claims 1characterized in that the vesicles of scrubbing liquid laden withcontaminating particles are eliminated at the head of the Venturi columnby impact on an impinger having walls, said impact causing saidscrubbing liquid to trickle along the walls of the impinger.
 5. In avertical-axis Venturi column useful for the purification of a gas (G)contaminated by liquid and/or solid particles, having, in its bottompart, at the throat (3) of a Venturi, means (19) for pneumaticatomization of a scrubbing liquid in a rising stream of the gas to bepurified, said means (19) injecting said scrubbing liquid in a directionsubstantially perpendicular to that of said gas stream to be purified,and in its top part, downstream of said throat (3), means (7) forelimination of the vesicles of scrubbing liquid entrained by said gasstream and laden with contaminants, the improvement which comprisesmeans (20) for generating a sound field at said throat, said sound fieldbeing intended to cause the contaminating particles to shift relative tothe vesicles of scrubbing liquid.
 6. A Venturi column according to claim5 characterized in that the means (20) for generating the sound fieldare upstream of the means (19) for atomizing the scrubbing liquid.
 7. AVenturi column according to claim 5 characterized in that thevesicle-eliminating means are impingers (7), the geometry of whichcauses the impacted vesicles to trickle along walls.
 8. A Venturi columnaccording claim 5 characterized in that the Venturi has a circular,plane cross-section perpendicular to the flow of the gas stream.
 9. AVenturi column according to claim 8 characterized in that it has anacoustic generator (20), fed with compressed air (212) and mounted in aresonance tube (211), said resonance tube acting as a nozzle which usessaid compressed air (212) as a driving gas injected into the throat (3)of the Venturi.
 10. A Venturi column according to claim 5 characterizedin that it contains one or more identical Venturis of rectangular, planecross-section perpendicular to the flow of the gas stream, said Venturisbeing mounted in parallel.
 11. A Venturi column according to claim 10characterized in that each Venturi has a series of acoustic generators(20) fed with compressed air (212) and mounted in a resonance tube(211), said resonance tube acting as a nozzle which uses said compressedair (212) as a driving gas infected into the throat (3) of the Venturi.12. A Venturi column according to claim 10 characterized in that eachVenturi is equipped with two vibrating-jet whistles (39); rods (36),which generate Karman vortices, advantageously being located upstream ofthe resonance volumes (37) of said vibrating-jet whistles (39).
 13. AVenturi column according to claim 12 characterized in that, upstream ofsaid whistles (39), it is equipped with means (34) for injecting adriving gas, said driving gas being mixed with the gas to be purifiedand at least partially compensating for the pressure losses generatedespecially by the sonic generator (39).
 14. A Venturi column accordingclaim 13 characterized in that the divergent portion (41) of eachVenturi has a cross-section which varies exponentially with the distancefrom the origin of said divergent portion (41) so as to act as anacoustic flare.
 15. A Venturi column according to claim 9 wherein saidacoustic generator (20) is a Galton whistle type (210) acousticgenerator.
 16. A Venturi column according to claim 11 wherein saidacoustic generator (20) is a Galton whistle type (210) acousticgenerator.